1. Field of the Invention
The present invention relates to methods of making foam insulated water heaters. More particularly, the present invention relates to a method of insulating a water heater tank with a novel, low viscosity expandable polyurethane foam insulation material.
2. Description of the Prior Art
The advantage of using rigid polyurethane foam insulation in water heater construction has been recognized for several years. For example, the heat conductivity of polyurethane foam is lower than that of fiberglass, thereby providing superior insulation properties. Thus, it is possible to obtain the same or better insulation properties utilizing a substantially reduced insulation wall thickness, as compared to conventional insulation materials, such as fiberglass. This results in more energy efficient water heaters having a reduced size thereby providing lower packaging and shipping costs.
In addition, the rigidity of the foam insulation, when compared with that of fiberglass, provides improved resistance to dents in the exterior jacket of the tank. This factor permits the use of less sophisticated (and therefore less expensive) shipping containers.
Although the superior insulating properties of expandable foam materials such as polyurethane have been well recognized for many years, the use of foam as an insulating material in water heaters has been to date quite limited. This is due at least in part to the production problems encountered using expandable foam materials resulting in higher production costs. One of the major problems associated with water heater manufacturing, and particularly the production of foam insulated water heaters, has been the method by which the foam insulation layer is formed about the tank. Generally, the foam is injected as a liquid which continually expands and eventually matures into a rigid foam layer. Usually the liquid foam is injected into the annular space between the inner tank and the outer jacket. Unfortunately, the liquid foam has a tendency to leak out of any small openings in the seams of the outer jacket. In addition, the sides of the tank generally are provided with openings such as a drain opening or with valves such as a temperature and pressure release valve. Also attached to the sides of the tank are electric controls and other components such as thermostats. In the past, it has been a serious problem in preventing these openings and components from becoming covered with foam, interfering with subsequent servicing, repair, adjustment, etc.
One way of shielding these areas from the expanding foam has been to pack the regions, between the tank and the outer jacket, surrounding the nipples and controls with fiberglass insulation material. The fiberglass insulation material then serves as a "foam dam" during the foaming operation.
Other processes utilize a plastic bag filled with the expanding foam material, the bag being positioned around the tank. In U.S. Pat. No. 4,372,028 the liquid foam material is first injected into a bag. The bag is then sealed and positioned circumferentially or longitudinally about the tank. The bag may have welded cut out openings which fit over any components attached to the exterior wall of the tank. Because the foam is wholly contained within an enclosed plastic bag, there is no danger of the foam leaking into unwanted areas. Once the foam within the bag expands sufficiently, it forms an effective dam about the electrical control or other area. Subsequently, the remaining annular space between the tank and the outer jacket is foamed.
U.S. Pat. No. 4,447,377 utilizes another type of plastic bag used in foam insulated water heaters. The bag has a shape which may extend substantially around the entire circumference of the tank. The bag is first positioned around the tank and then the outer jacket is positioned around the bag. Cut out weld holes may be provided in the bag and positioned about any components such as electrical controls, thermostats, drain lines, etc. The top pan is then positioned over the jacket and foam is injected, through a single opening in the top pan, into the bag. Since the expanding foam is wholly contained within the bag, there is no danger of unwanted foam leakage.
Unfortunately, the foaming bags or envelopes used in the prior art have encountered serious problems. In applications such as electric water heaters, the bags are typically used over only a portion of the tank surface. Those areas of the tank surface having electrical control components and other devices (such as thermostats, temperature and pressure relief valves and drain openings) are packed with blocks of conventional insulation materials such as foam or fiberglass. In these applications it is necessary to tailor the size and shape of the bag so that it precisely dovetails with these blocks of fiberglass or foam. This is difficult from both design and production standpoints since the positioning of the bag on the tank surface prior to foam injection becomes quite critical.
In addition, the enclosed bags or envelopes, typically composed of airtight materials such as polyethylene sheet, have a strong tendency to trap air in the corners of the bag, thereby forming voids when the foam eventually sets. The problem of foam voids becomes especially acute as the size of the insulation cavity is reduced. Additionally, the mass of the bag impedes the flow and expansion of the foam and prevents the foam from completely filling the cavity.
In addition, further problems are encountered when foam is injected through a single opening in the top pan. Due to the single opening, the injected foam is typically injected in a short single shot in a rather localized space between the tank side wall and the outer jacket. As the injected foam begins to expand, there is a tendency for the expanding foam to push the tank off center with respect to the outer jacket, and in some extreme cases to cause bulging to occur in the thin flexible jacket. Thus, it is an object of the present invention to provide a foaming method which will alleviate these localized high pressure areas caused by the expanding foam material.
It has now been discovered that the problem of localized pressure buildups can be alleviated by injecting the polyurethane foam material before the top pan is secured over the outer jacket. In this way, the shot of injected foam can be articulated around the annular space between the tank and the jacket. In addition, multiple single shots of the foam material can be injected without the necessity (and attendant costs) of providing a number of holes and corresponding plugs in the top pan.
It has further been discovered that the disadvantages associated with the prior art bags and envelopes may be overcome utilizing a foam composition having an unusually low initial viscosity, having high flow and gel indices and which generates lower foaming pressures than the prior art foams. It has also been discovered that using these new foams it is no longer necessary to use the foam restrictive bags and envelopes of the prior art which were so troublesome from the production and void formation standpoints.
Certain embodiments of the present invention may also utilize a novel sleeve configuration which wholly obviates the problems encountered with attempting to dovetail the enclosed bags and envelopes of the prior art against insulation blocks.